feat(https-outcalls): adopt the 0.2 HttpRequest builder, add a flexible outcall example

[eichhorl]

Overview

Two related changes to the HTTPS outcall examples.

1. Migrate to the 0.2 builder. ic-cdk-management-canister 0.2.0 removed the free http_request in favour of an HttpRequest builder, which always uses pricing version 2 (pay-as-you-go). This migrates send_http_get, send_http_post and daily_planner, bumps their crate pin to 0.2, and rewrites the comments that only held under version 1.

Each call also declares what it expects:

.with_expected_roundtrip_time_ms(10_000)
.with_expected_transform_instructions(1_000_000)

Unset expectations reserve the worst case: a 60-second round trip, and the full 5-billion-instruction query limit for a transform that only clears headers. Read from the replica's own cost_http_request_v2 on a 13-node subnet, that was 94% of the attached cycles:

	before	after
send_http_get	5,329,580,997	135,581,004
send_http_post	473,484,344	73,945,883
daily_planner	34,002,045,662	28,808,045,669

The two byte expectations stay at their default. They share one per-node budget with delivery of the result, so a response that exceeds them fails the call after the request has already been made.

2. Add send_http_flexible. A new example for flexible_http_request, where a committee of nodes each make the request and the canister receives their individual responses instead of one the subnet agreed on. It asks for the current time, which changes faster than nodes could agree on it, and tallies the distinct answers. The replication counts come from subnet_self_node_count(), because total_requests must not exceed the subnet size. There is no transform, since nothing has to be made identical across nodes.

Considerations

• Flexible outcalls are live. The feature flag flipped to Enabled on 2026-09-02 (c14df8a5b42), and both replica versions currently running across all 42 mainnet subnets contain it.
• send_http_flexible is Rust only, so it does not meet AGENTS.md's "ship both variants" for new examples: Motoko needs mops-ic 5.0.0, which is unpublished. AGENTS.md also asks for DX team agreement to maintain a new example. Happy to split it into its own PR.
• The 0.2 bump is scoped to the three migrated examples. The other 18 pinning 0.1.1 don't call the removed functions, and ^0.1.1 still resolves for them.
• daily_planner leaves max_response_bytes unset, so it still reserves for delivering 2MB and stays above its version 1 cost.
• dfinity/developer-docs quotes three regions from the migrated examples. The markers are unchanged, but its .sources/examples pin needs bumping after merge. The guide's "Flexible request" section has no code yet, so the new snippet gets wired up in that same follow-up.

Verification

cargo check --target wasm32-unknown-unknown clean on all four. All four deploy and call the live endpoints in ghcr.io/dfinity/icp-dev-env-rust:v2.2.1: send_http_get PASS, send_http_post PASS, daily_planner 6/6, send_http_flexible 2/2.